This bottom end is the foundation for the strongest engine I have or have had, if you. Lost power Im gonna say you have something going on besides the lower.
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I acttually thought about that also, the PK80 is a 40mm stroke if I remember right, the grube engines are a 38mm stroke im pretty sure.I don't know what the stroke of the pk80 is but does the piston travel all the way to the top of the cylinder? Or it could be compression related, I would check to make sure it didn't break a ring.
No offense intended bro, Im not new either and I can tell you your problem is likely nothing to do with the engine lower.When I say hardly runs. It won't idle at lower rpms, under 4k. It takes a lot more peddling to get started. I lost about 1500 rpms off the top speed and the torque is a lot less, small hills are now a big problem. While running it seems to be missing . The bike was pulling 9500 rpms 40+ mph. I'm not new to this by any means, I'm just stumped by what the issue could be. Thanks.